As sizes of transistors decrease, a thin film such as an insulating film which constitutes a sidewall spacer (SWS) of a gate electrode or the like may require a decrease in film forming temperature, an improved resistance to hydrogen fluoride (HF), and a decrease in dielectric constant. To this end, it has been considered to use, as an insulating film, a silicon boronitride (SiBN) film obtained by adding boron (B) to a silicon nitride film (SiN film) or a silicon borocarbonitride (SiBCN) film obtained by adding boron (B) and carbon (C) to a silicon nitride film (SiN film).
Such an insulating film is often formed by an alternating supply method that alternately supplies several kinds of process gases because high step coverage characteristics are required. For example, using a silicon (Si)-containing gas as a silicon precursor, a boron-containing gas as a boron precursor, a carbon-containing gas as a carbon precursor, and a nitrogen-containing gas as a nitrogen precursor, a SiBCN film can be formed on a substrate by performing a predetermined number of times a cycle that sequentially supplies those process gases to the substrate. However, the aforementioned method has a limit in enhancing the controllability of a composition ratio and the controllability of a film property.